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Real-time database systems can have security constraints in addition to timing constraints. Such real-time systems are typically contained in environments that exhibit hierarchical propagation of information, where mandatory access control for security is required. Conventional multi-level secure database models that implement mandatory access control are inadequate for time-critical applications and conventional real-time database models do not address security constraints. The objective of this work is to incorporate security constraints in real-time database systems in such a way that not only is security achieved, but achieving security does not degrade real-time performance significantly in terms of deadlines missed. We present concurrency control algorithm for secure real-time databases that the Secure OPT algorithm uses the properties of an optimistic concurrency protocol. We implement the algorithm and study their performance using a real-time database system simulation model. Our study covers both soft and firm real-time databases. Results show that the algorithm performs fairly well in terms of security and timeliness compared to a non-secure algorithm. We argue and show that achieving more security does not necessarily mean more sacrifice in real-time performance.